Synthesis and effect of metal–organic frame works on CO 2 adsorption capacity at various pressures: A contemplating review

Abstract

The most important environmental challenge that the world is facing today is the control of the quantity of CO₂ in the atmosphere, because it causes global warming. Increase in the global temperature results in greenhouse gas emission, interruption of the volcanic activity, and climatic changes. The alarming rise of the CO₂ level impels to take some serious action to control these climatic changes. Various techniques are being utilized to capture CO₂. However, chemical absorption and adsorption are supposed to be the most suitable techniques for post-combustion CO₂ capture, but the main focus is on adsorption. The aim of this study is to provide a brief overview on the CO₂ adsorption by a novel class of adsorbents called the metal–organic framework. The metal–organic framework is a porous material having high surface area with high CO₂ adsorption capacity. The metal–organic frameworks possess dynamic structure and have large capacity to adsorb CO₂ at either low pressure or high pressure due to its cavity size and surface area. Adsorption of CO₂ in the metal–organic framework at various pressures depends upon pore volume and heat of adsorption correspondingly. In this review, different synthesis methods of the metal–organic framework such as slow evaporation, solvo thermal, mechanochemical, electrochemical, sonochemical, and microwave-assisted synthesis are briefly described as the structure of the metal–organic frameworks are mostly dependent upon synthesis techniques. In addition to this, different strategies are discussed to increase the CO₂ adsorption capacity in the metal organic-framework.

Keywords

metal–organic framework synthesis method high and low pressure adsorption

Introduction

Carbon dioxide is the most noteworthy amongst all anthropogenic greenhouse gases. About 60% of the global warming is only because of the emission of CO₂.¹ According to the Intergovernmental Panel on Climate Change,² the environment may possibly be enhanced by up to 570 ppm of CO₂ in the 21^st century causing an increase of about 1.9°C in the universal temperature, and about 3.8 m rise in the sea level.³ It is very important to control the CO₂ concentration by reducing human activities. Generally, there are some strategies applied to decrease the emission of CO₂ into the environment by the efficient use of energy, by using hydrogen and renewable energy as an alternative to fossil fuels and developing new carbon capture technologies.⁴

Owing to the depletion of natural resources, there needs to be an alternative way that can meet the fuel requirements. So the world is moving towards more sustainable fuels to meet her demands. For example, CO₂ can be used to produce fuel for the future. In this regard, tremendous amount of research focus is on converting CO₂ to useful fuels such as the production of methanol, syngas, etc.⁵

Various techniques are utilized to capture CO₂, which are cryogenic distillation, amine absorption, and membrane gas adsorption processes. The capture and adsorption of CO₂ is a new concept, which is still under investigation. Different adsorbents silica, alumina, and activated carbon are being utilized for CO₂ capturing. Activated carbon has been extensively used because of its low cost, thermal stability, and less sensitivity. Zeolite, alumina, and silica have produced remarkable results in the CO₂ adsorption but all such systems require substantial amount of energy for the release of captured CO₂, which leads to low energy efficiency and high cost. CO₂ sensors are also used in many applications, such as clean energy technologies, engine exhausts, and chemical industries to detect CO₂ gas. Several types of CO₂ sensors that have been developed are infrared,^6,7 surface acoustic wave,⁸ solid electrolyte,⁹ capacitive,^10,11 and resistive sensors.^12,13 The major drawback of such traditional adsorbents is their low adsorption capacity and difficult regeneration process.¹⁴

As mentioned earlier, activated carbon is inexpensive and is not greatly affected by the humidity present in the gas in comparison to that of zeolite. Activated carbon is thermally stable and requires less energy for the regeneration process. However, they have some limitations as activated carbon shows low adsorption capacities than that of zeolite at low pressure and ambient temperature. With increasing temperature, the adsorption capacity of the activated carbon decreases.¹⁵

On the other hand, zeolite adsorbents show high potential for the CO₂ adsorption. But still, they have some restrictions as they are robustly affected by temperature, pressure, and the presence of moisture in feed gas. With the increase in the temperature, the CO₂ adsorption capacity of zeolite adsorbents decreases and this adsorption capacity increases as the partial pressure of CO₂ increases. Zeolite adsorbents are very sensitive to the moisture present in the feed gas. For the reason that it is greatly hydrophilic in nature, extensive drying of the feed gas is required earlier to the CO₂ capture¹⁵ and elevated regeneration temperatures are required,¹⁶ which is more than 573.15 K. The additional drying and high regeneration temperature are responsible for the extra cost, which extensively affects the adsorption applications of zeolite adsorbent¹⁵ as shown in Figure 1. That the CO₂ capturing material should be easily regenerable is the main challenge for CO₂ capturing technologies.

Figure 1.

Limitation of traditional adsorbents.

In this review paper, limitation of traditional adsorbents and progress in CO₂ adsorption by using the metal–organic framework (MOF) will be the focus. First, different methods used for the synthesis of the MOF are describe in detail and then CO₂ adsorption studies through the MOF at high pressure and low pressure are discussed. After this, strategies to enhance the CO₂ adsorption capacity are summarized as well. Stability of the MOF towards moisture, which is a biggest issue, will also be discussed. Although several review papers are already present on the synthesis of MOFs and the CO₂ adsorption through MOFs, this study is the updated review of past reviews that include recent papers up until now. This is an attractive option for readers working in the field of the CO₂ capture because this paper contains comprehensive information related to the adsorption of CO₂ through the MOF and also highlights an important challenge regarding the stability of the MOF.

Novel class of adsorbents

Metal–organic framework

MOFs have been identified as the most promising next-generation technology for the carbon capture. CO₂ is adsorbed on the surface of the MOF through both physisorption and chemisorption. This class of adsorbent is very important for gas separation applications and has gained much attention during the recent years. MOFs are formed by linking organic and inorganic building blocks formed by metal-based nodes in Figure 2.

Figure 2.

Synthesis scheme of the metal–organic framework.

MOFs are highly porous, thermally and mechanically stable, and have very high surface area for selective adsorption. The selection of metals and synthesis of organic ligands is the most significant feature of MOFs.¹⁴ Different metals and organic ligands will form MOF with a variety of structures and properties.¹⁴

MOFs are very dynamic in structure and utilized in a number of applications such as gas adsorption, gas separation, hydrogen storage, methane storage, CO₂ capture, and purification of flue gas (Figure 3).¹⁷

Figure 3.

Adsorptive removal through the metal–organic framework.

Some fabricated MOF’s and their applications are listed in Table 1.

Table 1.

Different applications of the metal–organic framework in industries.

MOF	Application	Reference
(In-imidazoledicarboxylate)-based RHO-ZMOF	As a catalytic material	Alkordi et al.¹⁸
Mg-MOF-74	Separation of methane (CH₄) from a carbon dioxide (CO₂) mixture	Britt et al.¹⁹
ZIF-8	As sensors	Lu et al.²⁰
Microperoxidase-11 immobilized into a (MOF)	Superior enzymatic catalysis	Lykourinou et al.²¹
mmen-Mg₂(dobpdc)	Separation of carbon dioxide from air and flue gas	McDonald et al.²²
Zinc-based metal–organic framework	Used in drug delivery	Burrows et al.²³
Core-Shell Pd@ IRMOF-3	Multifunctional catalysts for cascade reactions	Zhao et al.²⁴
Amino-functionalized Ti(IV)-based MOF	Visible-light photocatalysts for the treatment of Cr(VI)-contained wastewater	Wang et al.²⁵
Copper nanocrystals encapsulated in Zr-based metal–organic frameworks	CO₂ hydrogenation to nethanol	Rungtaweevoranit et al.²⁶
Zn-doped ZIF-67	As catalyst for the CO₂ fixation intocyclic carbonates	Zanon et al.²⁷

Other than these applications, the purification of natural gas is extremely imperative to enhance the fuel efficiency. Natural gas is an important fuel utilized in different industries, vehicles, and automobile. A number of techniques are in line, which can purify the gas. In industries, flue gases exit at elevated pressure and temperature. In this regard, the development of material with high gas adsorption capacities is very critical. MOFs are preferred in industries because they are highly porous and are resistant to high pressure.

MOFs are preferred for the CO₂ adsorption because of its highly porous structure and its resistance to high pressure.¹⁴ The cavity size of the MOF due to metal and ligands are larger than other adsorbents and owing to this it is known to be important for gas capturing or storage.¹⁴ The properties and functions of MOFs can be simply tuned for particular applications.¹⁴ This ability of MOFs is a major feature that distinguishes them from the activated carbon and zeolite.¹⁵

For efficient adsorbency, there is a need to design an adsorbent that integrates efficient capturing and regeneration ability. MOFs are preferred over other traditional adsorbents used for the CO₂ capturing because it usually requires less energy for regeneration without degradation in its performance.^14,28

MOF works on the basis of different mechanisms by which CO₂ binds within the framework.²⁹ Generally, two different mechanisms (physisorption and chemisorption) take place in the MOF based on the difference in the affinity of CO₂ to be adsorbed on the surface of MOFs.²⁹ Physisorption is the weak chemical bonding between CO₂ and adsorbent, which can be easily reversible by utilizing less amount of energy.²⁹

This physisorption occurs due to dipole forces present between the CO₂ molecule and weak nucleophilic/polar functional groups present in the MOF.²⁹ Usually at low pressure the process of chemisorption take place, as it is observed that there is a strong affinity between the CO₂ molecule and the MOF.²⁹ Chemisorption involves chemical interaction that is the formation of covalent bond at low pressure while physisorption occurs at high pressures.²⁹

MOFs are dependent on many parameters, which are temperature, compositions of solvent, reaction times, reagent ratios, concentrations, and pH of the co-solvent solution.³⁰ These parameters can play a vital role in the optimization and synthesis of different MOFs. Little change in any of the parameters can cause an adverse effect on the gas separation and gas storage applications.¹⁵ The properties and functions of MOFs can be simply tuned for particular applications.¹⁴ This easily tuned ability of MOFs is a major feature that can distinguish them from activated carbon and zeolite.¹⁵

To be a good adsorbent, MOFs must fulfill the following criteria.

Important criteria for the selection of MOFs

MOFs used for high CO₂ adsorption must satisfy the following important criteria:

High adsorption capacity of MOFs. The CO₂ equilibrium adsorption capacity is represented by its adsorption isotherm. The quantity of MOFs required can be calculated from the adsorption equilibrium capacity.³¹

High selectivity of MOF for CO₂. The adsorption selectivity of MOFs is defined as the ratio of the CO₂ capacity to the bulk of other gas components such as oxygen, nitrogen, etc.³¹

MOFs must exhibit fast adsorption and desorption that is higher than the adsorption rate.³¹

High strength during continual adsorption and desorption. As stability determines the life time of MOFs and the regularity of their replacement,³¹ the number of cycles must be increased.

Good mechanical stability³¹ of MOF, that is it must be able to bear humidity.¹⁵

Low cost.

MOFs has become an interesting group of material for researchers because their flexibility increases the scope of its application.³² MOFs can be prepared on large scale through low-priced or simple synthesis methods.³³ Different methods are being used for the synthesis of MOFs, such as solvo-thermal synthesis, microwave synthesis, and sonication synthesis.³⁴ However, solvothermal method is utilized mostly for the synthesis of MOFs.³⁵ Including this, thermal treatment is also done for the synthesis of MOFs. Other methods used ion-exchange treatment but this utilizes additional chemicals or more experimental steps.³⁶ All of these methods can produce MOFs of different sizes and morphologies that are used in different applications.^30,37 Considering all the facts in mind, it can be stated that synthesis methods are mainly responsible for attaining the uniform porous geometry of crystals.³⁸ Other than this, it also effects the adsorption capacity.³⁷

Various procedures for the synthesis of MOFs are described below.

Slow evaporation method

Slow evaporation method is the simplest and easy method from all the available synthesis method because there is no need for any external energy source.³⁹ Sometimes this method is preferred because it is a room-temperature process. But long synthesis time i.e. several hours to days is the main problem in this method.⁴⁰ In this procedure, a solution is prepared from metal and organic linker without using any external energy. The main drawback of this method is time consumption, which can be overcome by using solvents having low boiling point.^37,41 In Table 2, different metal source and organic linker, which form MOFs by slow evaporation method are mentioned.

Table 2.

Metal–organic framework (MOF) synthesis by slow evaporation.

Synthesis method	Metal source	Organic linker	MOF	Temperature	References
Slow evaporation	Zn(OAc)₂.2H₂O	2,5-Dihydroxyterephtalic acid	MOF-74	Room temp.	^34,36
	Zinc Salts (Zn) and Zinc SBU	H₂BDC, BDC (Terephthalic acid)	MOF-5	Room temp.	^34,36,37
	Zn(OAc)₂.2H₂O	Acetylenedicarboxylic acid	IRMOF-0	Room temp.	^34,36
	Ag₂O	H₂PHBPZ	CFA₃	Room temp.	³⁸

Solvothermal synthesis or hydrothermal synthesis

Solvothermal method is the most widely used synthesis method for MOF’s preparation.³⁹ MOF’s prepared by solvothermal method are represented in Table 3. In this synthesis process, reaction is carried out in a closed vessel at an elevated temperature and pressure for few hours or days.³⁰ For low temperature, glass vials are used and when the temperature is higher than 400 K Teflon-lined autoclaves are used.⁴⁵ Mostly organic solvents having high boiling point (acetone, ethanol, methanol, dimethyl formamide) are used in this method.³⁹ The following are the advantages of this procedure:

Table 3.

Metal–organic framework (MOF) synthesis by hydrothermal and solvothermal methods.

Synthesis method	Metal source	Organic linker	MOF	Temperature (K)	References
Hydrothermal	VCl₃	BDC	MIL-47	473.15	⁴³
	Al(NO₃)₃·9H₂O	NDC	MIL-69	483.15	⁴⁴
	Al(NO₃)₃·9H₂O	H₄BTEC	MIL-120	483.15	⁴⁵
Solvothermal Conditions	Zn(NO₃)₂·6H₂O	H₃BTB	MOF-177	373.15	^34,46
Solvothermal Conditions	FeCl₃·6H₂O	BDC	MIL-88	423.15	^47,48

One step synthesis.⁴⁶

Controlled morphology of crystal.⁴⁷

Mild temperature condition and short time requirement is the most efficient condition for synthesis.⁴⁶

Microwave-assisted synthesis

For MOF’s synthesis, this method is very fast because microwave-assisted method provides an efficient way of heating.⁵⁴ This method is mostly used in organic chemistry⁵⁵ and have been widely used for the preparation of nanosized metal oxides.⁵⁶ Different MOF forms by microwave-assisted method are given in Table 4.

Table 4.

Metal–organic framework (MOF) synthesis by microwave-assisted method.

Synthesis method	Metal source	Organic linker	MOF	Temperature (K)	Time (min)	References
Microwave-assisted synthesis	Cr(NO₂)₃·9H₂O	H₂BDC	Cr-MIL-101	483.15	40	⁶⁰
	FeCl₃·6H₂O	H₂BDC	Fe-MIL-53	423.15	10	⁶¹
	Cu(NO₃)₂·3H₂O	H₃BTC	HKUST-1	413.15	60	⁶²
	C₃H₉CrO₉S₃	H₃BTC	Cr-MIL-100	493.15	240	⁶³

Microwave-assisted method involves microwave energy to produce nanosized crystals when microwave energy is applied for about an hour.⁵⁷ A solution mixture within a suitable solvent is placed in a Teflon vessel, where the vessel is covered and placed in the microwave unit for appropriate time and temperature.⁵⁸ In the microwave unit, conversion of electromagnetic energy to thermal energy occurs, that is microwave energy is coupled with dipole moment of the precursors causing molecular motion because of the rapid heating of the solution mixture.⁵⁸ This resulted in nanosized crystals of uniform size.⁵⁹ This method is also termed as “microwave-assisted solvothermal synthesis”.⁵⁹ The following are the advantages of this synthesis method

Fast crystallization⁶⁰

Phase selectivity/high purity⁶¹

Identical morphology/controlled morphology⁶²

Narrow particle size distribution/uniform size distribution⁶³

Sonochemical synthesis

Recently, sonochemical synthesis has been effectively being used for the quick preparation of MOFs because it reduces the crystallization time by using ultrasonic radiation.⁶⁸ Ultrasound is the mechanical vibration having frequency of 20 kHz to10 MHz.⁶⁹ By interaction of high energy ultrasound radiation with liquids, molecules causes chemical change.⁷⁰ These high energy radiation enhancing the physical and chemical changes in a solution is due to a cavitation process.⁷¹ The term cavitation is the formation and instantaneous collapse of bubbles occurring in the solution after sonication.⁷² This causes extremely fast release of energy having high temperature of about 4000 K or 5000 K and high pressure of 1000 bar^73,74 producing fine crystallites.⁷⁵ MOFs synthesis by sonochemical method is listed in Table 5. The advantages of this procedure are

Table 5.

Metal–organic framework (MOF) synthesis by sonochemical method.

Synthesis method	Metal source	Organic linker	MOF	Conditions	References
Sonochemical synthesis	Zn(NO₃)₂·6H₂O	H₂BDC	MOF-5	60 W, 30 min	⁷⁵
	Mg(NO₃) 6H₂O	H₄DHBDC	Mg-MOF-74	500 W, 60 min	⁷⁶
	Cu(OAc)₂·2H₂O	H₃BTC	HKUST-1	150 W, 60 min	⁷⁷

Energy efficient/environment-friendly method to generate homogeneous nucleation centers as there is no direct interaction between ultrasound and molecules.^76,77

Reduces crystallization time.⁷⁸

Significantly smaller particles size than those by the conventional solvothermal synthesis.⁷³

Producing very fine crystallites.⁷⁵

Mechanochemical synthesis

Mechanochemical synthesis is the simplest method than the other method for MOF’s synthesis.⁸² In this method no solvent is used for MOF’s synthesis, and mechanical force is applied by means of mechanical ball mill.⁸³ When mechanical force is applied on the mixture of metal salts and organic linkers, chemical transformation take place at room temperature,⁸⁴ that is the mechanical breakage of intramolecular bonds and formation of new bonds. In the past, mechanochemistry is mostly used in synthetic chemistry. MOFs synthesis by mechanochemical method is shown in Table 6.

Table 6.

Metal–organic framework (MOF) synthesis by mechanochemical method.

Synthesis method	Metal source	Organic linker	MOF	Conditions	References
Mechanochemical synthesis	Cu(OAc)₂·H₂O	H3BTC	HKUST-1	25 Hz, 15 min	⁸⁵
	ZnO	HMeIm	ZIF-8	5–60 min, 30 Hz	⁸⁶
	ZnO	Him	ZIF-4	5–60 min, 30 Hz	⁸⁶

In 2006, first MOF synthesis by this method was reported.⁸⁵ Now, this method is used for MOF’s synthesis on large scale.⁸² Some advantages of this method are listed below.

Organic solvents can be avoided⁸⁶

Room temperature is sufficient⁸⁷

Side products formed are harmless⁸⁸

MOF can be obtained in short reaction time⁸⁶

Electrochemical synthesis

Recently, electrochemical method that is a rapid synthesis method was used for reproducible production of large amount of MOF crystals.⁹¹ In 2005, the electrochemical synthesis of HKUST-1 MOF was first reported.⁹² In electrochemical method, metal salts are replaced by metal ions.⁹³

The electrochemical synthesis of MOFs uses metal ions continuously supplied through anodic dissolution as a metal source. Metal ions continuously provided through anodic dissolution reacts with organic linker molecules and electrolyte in the reaction medium where large amount of MOF crystals can be obtained.⁹³ MOF synthesis by electrochemical synthesis method is represented in Table 7. Advantages of this synthesis procedure are

Does not require metal salts⁹¹

Offers continuous production of MOF crystals⁹³

Faster synthesis at lower temperatures than conventional synthesis⁹¹

Table 7.

Metal–organic framework (MOF) synthesis by electrochemical method.

Synthesis method	Metal source	Organic linker	MOF	Conditions	References
Electrochemical synthesis	Al(NO₃)₃·9H₂O	H₂BDC	Al-MIL-53	Electrolyte: KCl, 363.15 K, 10 mA	⁸⁷
Electrochemical synthesis	Al(NO₃)₃·9H₂O	H₂BDC-NH₂	Al-MIL-53-NH₂	Electrolyte: KCl, 363.15 K, 10 mA	⁸⁷

MOF synthesis includes selection of the method by which MOFs can be prepared, required time, type of energy supplied, and required temperature. These are optimized conditions for the preparation of MOFs by different methods. Keeping all the above mentioned synthesis process, collection of various parameters is represented in Table 8.³⁹

Table 8.

Summary of different methods and conditions for MOF synthesis.

Method	Solvothermal	Microwave	Sonochemical	Slow evaporation	Electrochemical	Mechanochemical
Energy	Thermal energy	Microwave radiation	Ultrasonic radiation	No external energy	Electrical energy	Mechanical energy
Time (min)	2880–5760	4–240	30–180	10,080–306,600	10–30 min	30–120
Temperature (K)	353–453	303–373	273–313	298	273–303	298

CO₂ adsorption at various pressures by MOFs

Adsorption of CO₂ through MOF is very economical as well as recycling of MOFs is easily possible.¹⁴ There are many important aspects related to the CO₂ adsorption through the MOF. However, adsorption capacity is one of the important parameters for the adsorption of CO₂ in MOFs. The adsorption capacity refers to the gravimetric capacity or volumetric capacity of CO₂. The maximum adsorption of CO₂ is mostly done at room temperature and high pressure.⁹⁴

The adsorption capacity for MOFs at room temperature and low pressure are mostly controlled by chemical characteristic of the pore surface and heat of adsorption of gas.¹⁵ While adsorption isotherms at elevated pressure are influenced by the surface area, structure of cavity adsorption selectivity, the heat or enthalpy of adsorption, and mechanical strength of MOFs.¹⁵ The CO₂ adsorption capacity at low pressure depends on heat of adsorption for CO₂ to be adsorbed in the MOF.⁹⁵

Adsorption of CO₂ by MOFs can be done at various pressures, which are distinguished as low pressure and high pressure. It is essential to study the effect of CO₂ adsorption by MOFs at low- and high-pressure conditions because at both pressures the contact time of CO₂ with MOFs varies. In Table 9, MOF’s listed are used for low-pressure condition and MOF’s used for high-pressure conditions are shown in Table 10.

Table 9.

Low-pressure MOFs.

Material	Name	BET (surface area, m²/g)	Temperature (K)	Pressure (bar)	Capacity (wt%)	References
Cu₂(bptc)(H₂O)₂(DMF)₃	MOF-505	1547	298	1.1	12.6	⁹²
Zn₄O(BTB)₂	MOF-177	5400	298	1	3.6	¹¹²
Zn/DOBDC		816	296	1	5.8	¹⁰⁴
Mg₂(dobdc)	Mg-MOF-74, CPO-27-Mg	1174	298	1	27.5	¹¹³
Zn₄O(PDC₎₃	IRMOF-11	2096	298	1.1	7.3	⁹²
Cr₃O(H₂O)₂F(BDC)₃	MIL-101(Cr)	2674	319	1	4.2	¹¹⁴
Al(OH)(2-amino-BDC)	NH2-MIL-53(Al),USO-1-Al-A	960	298	1	12	¹¹⁵
Zn₄O(BDC)₃	MOF-5, IRMOF-1	2304	296	1	8.5	¹¹⁶
			295	1	9.24	⁹²
Zn₄O(BDC-NH₂)₃	IRMOF-3	2160	298	1.1	5.1	⁹²

Table 10.

High-pressure MOFs.

Material	Name	BET(surface area, m²/g)	Temperature (K)	Pressure (bar)	Capacity		References
Material	Name	BET(surface area, m²/g)	Temperature (K)	Pressure (bar)	wt%	mmol/g	References
Zn₄O(BBC)₂(H₂O)₃	MOF-200	4530	298	50			¹¹⁰
Zn₄O(BBC)₂(H₂O)₃	MOF-200	4530	298	50	73.9		¹¹⁰
Zn₄O(BDC)₃	MOF-5, IRMOF-1	2296	298	35		21.7	⁹²
Mg₂(dobdc)	Mg-MOF-74	1542	278	36	68.9		¹¹⁷
Al(BDC)(OH)	MIL-53(Al)	1100	298	25		10	¹¹⁸
			304	25	30.6		¹¹⁸
Al(ABDC)(OH)	Amino-MIL-53(Al)		303	13	30	6.7	¹¹⁹
Cr(OH)(BDC)	MIL-53(Cr)	1100	304	25		10.1	¹¹⁸
Cr₃O(H₂O)₂F(BDC)₃	MIL-101(Cr)	4230	304	50	56.9	40	¹⁰⁶
Zn₄O(BDC)(BTB)_4/3	UMCM-1	4100	298	24	52.7		¹⁰⁸
Zn₄O(BTE)_4/3(BPDC)	MOF-210	6240	298	50	74.2		¹¹⁰
Zn₄O(DBDC)₃	IRMOF-6	2296	298	40		19.8	¹⁰⁰
Zn₄O(BTB)₂	MOF-177	4500	298	50	60.8		¹¹⁰

According to Tables 9 and 10, Millward and Yaghi⁹⁶ found that the structure, surface area, and pore dimension of MOFs affect the CO₂ adsorption capacities, such as MOF-2,⁹⁷ MOF-505 and Cu₃(BTC)₂),^98,99 MOF-74,¹⁰⁰ IRMOF-11,¹⁰¹ amino and alkyl-functionalized pore IRMOF-3 and -6¹⁰¹ and additional high porosity frameworks IRMOF-1 and MOF-177.^102,103 They investigated that MOF-74,¹⁵ MOF-505, and Cu₃(BTC)₂ had the maximum capacities at low pressure (1 bar).^15,96 Yazaydin and Snurr¹⁰⁴ found that MOFs have the highest CO₂ adsorption capacity at 0.101325 bar.

MOF-177 is made of zinc and a bulky linker 4,¹⁴ 40, 400-benzene-1,3,5-triyl-tribenzoic acid (H₃BTB) shows maximum CO₂ adsorption capacity of 33.5 mmol/g at elevated pressure of 35 bar and 298.15 K, which is higher than any of the known adsorbent material at that time.¹⁵ This great CO₂ adsorption capacity is due to the large pore space present in MOF-177. At low-pressure MOF-177, the adsorption capacity is increased gradually with the rising pressure;¹⁵ however, at 40 bar, there is a pointed increase in the adsorption isotherm.¹⁵ This adsorption isotherm action was noted in several MOFs as in MIL-53, [Cu(bpy)-(BF₄)₂(H₂O)₂].(bpy),¹⁰⁵ Ni(bpy)₃(NO₃)₄,¹⁰⁶ and IRMOF-1.¹⁰⁷

But in recent times,¹⁵ the MOF (Mg-MOF-74, Mg/DOBDC) was prepared by Caskey et al.,¹⁰⁸ which shows highest CO₂ adsorption than MOF-177. The Mg-MOF-74 shows the highest adsorption capacity for CO₂ (23.6 wt%, 5.36 mmol/g) at 0.1 bar and ambient temperature. The adsorption capacity is larger than zeolite, and is assumed to be a good adsorbent for CO₂ separation and provided the CO₂ adsorption capacity of 4.7 mmol/g (20.7 wt%) at 1.01325 bar and 298 K.¹⁰⁹

According to the literature, most efficient type of MOF for CO₂ adsorption is MIL-101 (Cr),¹⁴ which has a capacity of 40 mmol CO₂/g or 390 cm³ (STP)/cm³ than that of MOF-177 and have an adsorption capacity of 33.5 mmol CO₂/g or 320 cm³ (STP)/cm³. Furthermore, MIL-101 (Cr) was activated by using ethanol and NH₄F to increase the surface area and pore volume, which leads to a 40 mol kg^-1 CO₂ adsorption capacity at 50 bar and 304 K.¹¹⁰ From the literature,¹⁷ it was found that the activation method plays an important role in the CO₂ adsorption capacity and attraction towards CO₂ and different methods to activate results in unusual CO₂ loadings and heat of adsorption.¹⁷

Koh et al.¹¹¹ synthesized the UMCM-1 MOF by zinc nitrate, terephthalic acid, and 1,3,5-tris(4-carboxyphenyl)-benzene. UMCM-1 MOF has a large surface area (4100 m² g¹) and large pore volume (2.141 cc g¹). Mu et al.¹¹² found the CO₂ adsorption isotherms at different temperatures for UMCM-1 MOF and found the CO₂ adsorption capacity of 23.5 mol kg⁻¹ at 24 bar and 298 K.

Farha et al.¹¹³ designed NU-100 MOF by using the computational modeling, where NU-100 is made up of copper metal centers and hexatopic carboxylate ligand (LH₆). They have a high surface area (6143 m² g⁻¹) and CO₂ uptake of 46.4 mol kg⁻¹ at 40 bar and 298 K.¹⁴

Recently, Furukawa et al.¹¹⁴ synthesized some different types MOFs that have extra-high porosity. MOF-210 shows the largest BET surface area of 10,400 m² g⁻¹ and pore size that are reported so far. The extra-high porosity is achieved by increasing the amount of organic linkers by means of terephthalic acid¹⁴ to 4, 40, 400-(benzene-1,3,5-triyl-tris (benzene-4,1-diyl)) tribenzoate in MOF-200. MOF-200 and MOF-210 achieved the CO₂ adsorption capacity of 54.5 mol kg⁻¹ at 50 bar and 298 K.¹¹⁵

At elevated pressures, the CO₂ adsorption capacity depends on the surface area and the pore size of MOFs.¹⁴ Increasing the surface area and pore volume of MOFs is an efficient means to increase their CO₂ storage capabilities.¹⁴ Many of the bulky organic linkers with several benzene rings were used to produce MOFs with an ultra-huge surface area and pore volume.¹⁴

Strategies to enhance the MOF adsorption capacity

Pore size is significant for the CO₂ adsorption, as little change in pore volume causes a remarkable change in the adsorption. The CO₂ adsorption uptake for MOFs can be enhanced and modified by decreasing the pore volume, such as interpenetration or catenation.¹⁴ Keskin and Sholl¹²⁴ found that the CO₂ uptake is enhanced owing to the effect of additional little pores and adsorption sites created by the interpenetration of framework. The presence of hydrogen-bond interface between a CO₂ and MOF can enhance the CO₂ adsorption.¹⁴ Mu et al.¹²⁵ showed that the addition of the electron-donating groups into an organic linker would greatly enhance the adsorption capacity of MOFs for CO₂ and CH₄ mixture separation. This improvement becomes more prominent with the enhancement of the electron-donating group causing no effect by the addition of electron withdrawing groups.

This shows that the CO₂ molecule acts as an electron-acceptor only in the electron donating–accepting process.¹²⁶ The functionalization of MOFs will enhance the CO₂ uptake capacity.¹⁵ This will also increase the selectivity by grafting a functional group that have high affinity for CO₂ such as arylamine,¹⁰⁴ alkylamine, and hydroxyl group. By amine functionalization, the chemical bonding can also be introduced with CO₂ molecule into MOFs. MOFs can be functionalized by amine and its derivatives.¹⁴ This functionalization can be made during or after the synthesis processes.¹⁴ This functionalized framework preferentially uptakes CO₂ at low pressures. The amine-functionalized MOF shows a large adsorption of CO₂ at very little pressures than that with the nongrafted material.

By flexible hexacarboxylate ligand with amide linking groups,¹⁶ Zheng and Bai¹²⁷ formed a highly porous rht-type new MOF. This MOF shows high surface area, a high enthalpy of adsorption, and large CO₂ gas adsorption capacity. This indicates that the functionalization of MOF with acrylamide can appreciably increase the CO₂ adsorption capacity and selectivity of MOFs. Type of metal is very important for modification and optimizing the adsorption properties of MOFs.¹⁵ A number of new MOFs can be formed by varying the metal ions (k, Cu, Zn, Cr).¹⁵

Through electrostatic forces, CO₂ adsorption capacity can be enhanced. These electrostatic forces are incorporated into MOF during doping of metal ions and by means of polar species variation. The existence of additional framework ions can improve the interactions between the guest molecules, which acts as extra adsorption sites.¹²⁸

Xiang et al.¹²⁹ found that the as a result of doping¹⁴ carbon nanotube tailored HKUST-1 with Li, the CO₂ and CH₄ adsorption capacities are improved. To get fine improvement, Li has to be maintained at small concentration as high Li doping causes the distortion of the framework. Also, post-synthetic process becomes successful means to give novel properties or modify existing properties of MOFs to meet the particular requirement.¹⁴

Studies have been conducted about the synthesis of MOFs for CO₂ adsorption and capture, while ignoring an important aspect of the actual composition of flue gas affecting the performance of MOFs as the actual flue gas is saturated with moisture of about 5–7 vol.%²⁹

Effect of water on MOFs

Stability of MOFs towards moisture is the biggest challenge as MOFs lose their structure completely when exposed to the humid environment such as removing CO₂ from the flue gas. Removal of moisture from the flue gas increases the cost and is mostly impractical on a large scale, so it is extremely important that MOFs show stability towards moisture.¹⁷

The chemical and thermal stability of the MOF are usually less than that of other inorganic adsorbents because of the presence of weak interaction between metal and organic ligands. This is the major reason that MOFs are mostly moisture sensitive.²⁹

The MOF contains bond between metal and ligand, which is hydrolyzed when MOFs come in contact with water. It was found that MOF-5 has hydrophilic nature and on exposure to moisture it reduces its crystallinity.¹³⁰ The framework formed due to the strength of bond between metal and ligand is very important for the prediction of stability of the MOF in the presence of water.³² The strength of the bond is dependent on the pKa values of the organic ligand.¹⁷ Different approaches were used to increase the stability of the MOF such as functionalization of hydrophilic MOF with nitrogen containing organic linker after synthesis and using high valence metal ions as shown in Figure 4.^14,131

Figure 4.

A schematic representation of the stability of MOFs.

A simple approach to reduce humid effects on the MOF stability or CO₂ adsorption is to synthesize the MOF that can abhor H₂O effect, which means to make hydrophobic surfaces in MOFs.¹⁴ This can be done by the fabrication of MOFs utilizing hydrophobic surfaces and modification of synthesized hydrophilic MOFs.¹⁴

Although same synthesis methods are used for the preparation of hydrophobic MOF, post modification has been done to make the MOF as hydrophobic.¹⁴ This post modification/physical incorporation of functional groups into MOFs is basically developed for the functionalization of MOF, which can be made after synthesis processes.¹³²

The physical functionalization/incorporation of N–H groups such as amine into MOFs is suitable and easy method to synthesize hydrophobic-based MOF.¹³² Lin et al.¹³² used PEI for the impregnation of MIL-101. Activated MOF MIL-101 was slowly added to the solvent containing PEI with continuous mixing and then drying of this mixture was done at room temperature.

Bae et al.¹³³ functionalized Ni-based DOBDC with pyridine. This modification showed that pyridine changed the water loving affinity of the inner surface to the hydrophobic nature. This reduced the effect of water absorption without affecting the CO₂ adsorption capacity. Fracaroli et al.¹³⁴ introduced the modification in IRMOF-74-III with primary amine and further utilized this functionalized MOF for CO₂ capturing in the environment having 65% relative humidity.

Hydrophobic MOF²⁸ and its recyclability are the best alternative for capturing of CO₂.^28,134 Industrially, recycliability of the MOF is very important for the number of applications.¹³⁴ However, the recyclability and hydrophobicity in the MOF need to be explored further in future.¹³¹

Conclusion

In this review, adsorption of CO₂ by MOF’s is summarized. Currently, MOFs are in limelight because of its extraordinary textual properties, and its structural properties can be tuned by making simple modification in the synthesis method according to the application. One of the main advantages of MOFs than other traditional adsorbents is their diverse crystal structures and composition. In this review, we discussed different methods, which are used for the synthesis of MOFs at faster rate with bulk amount. Choice of metal ion source and organic linker makes it possible to design new compounds. Adsorption of CO₂ depends upon the pressure of feed gas. At elevated pressure, the adsorption capacity of CO₂ increases with the increase of the surface area and pore volume. Although in case of low pressure, the adsorption capacity of CO₂ is based on the heat of adsorption for CO₂ absorbed in MOFs.

It is possible to increase the CO₂ adsorption capacity or making MOF stable especially under moist condition by adding functionality inside the cavities of MOFs. By amine functionalization or metal ion exchange, CO₂ adsorption capacities are improved. Almost a number of various types of MOFs are present but MOF having high hydrothermal stability should be preferred in industries.

Footnotes

Acknowledgements

We like to acknowledge the support of MEMAR Lab at SCME, NUST, Pakistan.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Ayesha Rehman completed her MS degree in Chemical Engineering from the National University of Sciences and Technology (NUST). She is currently enrolled in PhD in Chemical Engineering at the National University of Sciences and Technology (SCME, NUST). Her area of interest is gas separation technologies, nanotechnology, and adsorption.

Sarah Farrukh completed her PhD in Membrane Technology and is now working as Assistant Professor at the School of Chemical and Materials Engineering (SCME), National University of Sciences and Technology (NUST). She is teaching undergraduate and postgraduate courses at the Chemical Engineering Department in addition to her administrative duties as PG program coordinator. She has been independently working on different research projects and supervising many undergraduate and post-graduate students. She is in-charge of the membrane research lab (MEMAR Lab). She has published more than 25 papers in ISI indexed journals. She has also published one book chapter with Springer. Her research area is membrane technology, gas separation, water purification, 2-D materials and CO2 capture, utilization and storage.

Arshad Hussain completed his MS and PhD in Chemical Engineering from Germany. As a Postdoctoral Fellow at Center for Marine CNG, St. Johns Canada, he worked on a research project to study the natural gas compositional, thermo-physical and fluid phase analysis during loading and unloading operations. He has also worked as senior researcher with State Oil Norway and Petronas Malaysia on the process design and feasibility analysis of CO2 Capture from Natural Gas/Flue Gases by using polymer membranes. His extracurricular activities are worth mentioning for being the Associate Editor NJES, conference secretary CEMP, Member, European Membrane Society and American Institute of Chemical Engineers besides being the Patron of NUST Literary Society.

Erum Pervaiz is working as Assistant Professor at Chemical Engineering Department, NUST. She has diverse research experience in materials development related to various applications. She is currently working on catalysis (Photo catalysis and electro-catalysis) and in the fields of water splitting for clean fuel productions.

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Synthesis and effect of metal–organic frame works on CO 2 adsorption capacity at various pressures: A contemplating review

Abstract

Keywords

Introduction

Novel class of adsorbents

Metal–organic framework

Important criteria for the selection of MOFs

Slow evaporation method

Solvothermal synthesis or hydrothermal synthesis

Microwave-assisted synthesis

Sonochemical synthesis

Mechanochemical synthesis

Electrochemical synthesis

CO2 adsorption at various pressures by MOFs

Strategies to enhance the MOF adsorption capacity

Effect of water on MOFs

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

CO₂ adsorption at various pressures by MOFs